For the afforestation of dry land that does not serve as farmland due to insufficient water retentivity of soil, soil of dry land has been actively improved so as to have water retentivity. Primary conditions for improving soil of dry land into soil suitable for plants include retentivity of water of a certain amount as well as blockage of salt contained in ground water. Further needed is a soil structure that can be improved as easily as possible and at lower cost.
In view of these conditions, there has been conventionally proposed an artificial soil structure that facilitates improvement into water retentive soil at low cost by applying water repellent treatment to particles such as sand and forming a water shield layer made of the water repellent sand (see JP 06-113673A). FIG. 35 depicts the artificial soil structure proposed in JP 06-113673A. This artificial soil structure includes a water shield layer 202 made of hydrophobic particles at predetermined depth from the ground surface in the soil, more specifically, depth allowing plants to sufficiently take root, and a water retentive soil layer 201 that is provided on the water shield layer 202 reserves water, so as to form soil not allowing water to flow to reach under the soil. Furthermore, the water shield layer 202 is expected to block salt contained in ground water. An original soil layer is denoted by reference numeral 203.
However, in this soil structure, the water shield layer 202 that may not be able to bear water pressure may be broken, or root rot may occur to plants due to excessive water, with relatively large rainfall being supplied in a short period of time in a rainy season on dry land, for example. Once the water shield layer 202 is broken, salt contained in ground water rises through the broken portion, with a result that the water shield layer 202 cannot prevent salt damage. Furthermore, in this structure, it is impossible to preliminarily specify a location of breakage in the water shield layer 202 or situations of breakage (such as the diameter of the broken portion). Analysis of the situations for mending and investigation for the appropriate mending method need to be conducted after the water shield layer 202 is broken, thereby requiring time or effort. Furthermore, it is difficult to mend the broken portion in the water shield layer 202 depending on its position (in cases where a plant or a building is positioned right above the broken portion).
On the other hand, a building such as a dam designed for reserve of water is provided with drain holes used for discharging water in order to reduce water pressure when the reserved water exceeds a certain amount. Even in the case of forming the water shield layer made of hydrophobic particles, a soil structure proposed in JP 06-62666A can be designed to have a configuration for discharge of water so as not to allow water to exceed a certain amount. FIG. 36 depicts a soil structure according to JP 06-62666A that includes a water shield layer 205 made of water repellent sand and provided in original soil basically similarly to JP 06-113673A, a water retentive soil layer 204 provided on the water shield layer 205 as improvement, and a portion (hereinafter, ordinary soil drain hole) 207 made of ordinary soil (soil with no water repellent treatment) partially provided in the water shield layer 205 and penetrating the water shield layer 205. An original soil layer is denoted by reference numeral 206. JP 06-62666A discloses the soil structure of FIG. 36 as the penetrating portion used for growing plants. The soil structure additionally includes the configuration of the ordinary soil drain hole 207 for allowing water to flow to reach under the water shield layer 205. In this configuration, even if relatively large rainfall is supplied in a short period of time, excessive water flows downward without overflowing onto the ground surface, breaking the water shield layer 205, or causing root rot. Furthermore, it is possible to specify the flow of water, so that mending for changing the amount of water flowing downward through the ordinary soil drain hole 207 can be planned easily and preliminarily.
However, in the soil structure according to JP 06-62666A, even in a season such as a dry season of relatively small rainfall, when water is desired to be reserved without any loss, water may flow through the ordinary soil drain hole 207. Furthermore, salt rises through the ordinary soil drain hole 207, thereby failing to prevent salt damage. There is required a soil structure that reliably prevents breakage of the water shield layer and salt damage while securing retentivity of water of a certain amount.
Many districts in the world cannot expect constant rainfall throughout the year but have seasons of relatively large rainfall and of relatively small rainfall. For example, as to annual rainfall in Kumasi in Ghana, the season of large rainfall from May to June has rainfall of 150 mm or more in one month, whereas the season of small rainfall from December to January has rainfall of 50 mm or less in one month. It is thus necessary to form a soil structure that can reserve water as much as possible in such a season of small rainfall and discharge excessive water in such a season of large rainfall, as well as constantly prevent rise of salt.